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Homework Help: Unsure about formula for kinetic energy

  1. Nov 20, 2015 #1

    Imagine the flagpole and ball with mass m fall together. Can I here use the formula mg*l*sin(theta)=(1/2)*mv^2
    If I can, in what direction will the velocity vector be - is it gonna be tangential to the flagpole?
  2. jcsd
  3. Nov 20, 2015 #2


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    You can use the formula, if

    1) the mass of the pole can be neglected and
    2) the diameter of the ball is much smaller than the length of the pole.

    The velocity vector would be perpendicular to the pole (but I think that's what you ment).
  4. Nov 20, 2015 #3
    "the diameter of the ball is much smaller than the length of the pole."
    Why is this?
  5. Nov 20, 2015 #4
    Your equation is incorrect. What does it tell you when theta is zero? Try again.

    Also, what idiot made up this problem. The US flag should never be shown disrespectfully falling down!!!!

  6. Nov 20, 2015 #5


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    For the same reason as discussed yesterday. This formula couldn't be applied for the pole because each slice dm moves with a different velocity depending on the position. If the diameter of the ball would have a similar value as the length of the stick, then you would have to take the different slices of the ball into account. If the stick is much longr, than you can consider all slices of the ball moving with the same velocity.
  7. Nov 20, 2015 #6
    Alright, thank you, I think I get the idea now
  8. Nov 20, 2015 #7
    It tells me there is zero potential energy at that point, everything is turned into kinetic energy?
  9. Nov 20, 2015 #8
    According to your equation, the velocity is zero when theta is equal to zero.
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